1. Field of the Invention
The present invention relates to a private branch mobile communication system, and more particularly to a private branch mobile communication system for a local area switched network which includes a plurality of private branch exchanges (PBXs) installed over a wide area and allows origination and termination of a call also when a radio terminal which belongs to a certain home PBX is used in a radio area managed by another PBX which is physically different from the home PBX and a private branch mobile communication method for the private branch mobile communication system.
2. Description of the Related Art
In recent years, in response to a demand for increase in area, variety and so forth of information exchange in an enterprise, for example, construction of a local area switched network wherein a plurality of PBXs are interconnected by a trunk line to allow private branch telephone exchange over a wide area as well as introduction of a private branch and public mobile communication system called PHS (Personal Handyphone System) for a place of business and public wherein radio base stations are connected to each PBX and radio terminals are accommodated in each radio base station are being popularized.
In such a private branch mobile communication system as described above, a plurality of radio base stations are distributed in a local area of an enterprise and radio terminals present in radio areas of the radio base stations are connected to PBXs through the radio base stations to allow extension communication in the same PBX, extension communication with a terminal in another area over a local area switched network through a dedicated line and line wire communication over a public network connected by a main wire.
In the conventional private branch mobile communication system described above, in order to allow a radio terminal which belongs to a PBX in a certain region to be used in a radio area managed by a PBX of another region, PBX roaming processing must be performed. This is because a system ID (SYS-ID) which is a system call sign provided in order to establish matching between a radio terminal and a system which provides a mobile communication service to the radio terminal is different among different PBXs and because, according to the specifications of the conventional private branch mobile communication system, even if a common communication protocol is used between a radio base station and a radio terminal, the radio terminal and the radio base station which have different SYS-IDs from each other cannot communicate with each other. This is equivalent to the fact that, in mobile communication services of public networks, a radio base station and a radio terminal whose undertakers are different from each other cannot communicate with each other.
In this manner, since a conventional PBX cannot control radio terminals other than radio terminals whose attribute information such as operation and management information is registered in the PBX itself, in order to identify those radio terminals, a SYS-ID is provided, and mobile communication is allowed only between a radio terminal and a radio base station who have the same SYS-ID. Accordingly, even in a private branch network in the same enterprise, SYS-IDs different from each other are provided to different PBXs which form the network.
Roaming processing proceeds in the following manner. When power supply to a certain radio terminal is made available in a radio area managed by a PBX (called visitor PBX) different from a PBX (called home PBX) in which attribute information of the radio terminal is registered, it receives a SYS-ID provided to the visitor PBX and being announced from a radio base station connected to the visitor PBX. If the radio station discriminates that the SYS-ID announced is different from that of the home PBX, then it changes over the SYS-ID to that one of the other SYS-IDs registered in advance for roaming which is same as the SYS-ID announced at present and then issues a position registration request. The visitor PBX receives the position registration request through the radio base station and discriminates the home PBX from the identification number of the radio terminal, and requests the home PBX for a registered attribute of the radio terminal and transfers the position registration information. The visitor PBX receives the registered attribute of the radio terminal from the home PBX and performs call control regarding the radio terminal as a temporary registered subscriber of the visitor PBX.
A technique which allows, in a private branch mobile communication system wherein a plurality of PBXs which form a local area switched network perform attribute registration of radio terminals independently of each other, appropriate roaming to be always performed including also information between tenants in such a case that one PBX accommodates a plurality of enterprises is disclosed in Japanese Patent Laid-Open No. Hei 9-247741.
However, where conventional PBXs are employed, management and operation information necessary for exchanging processing such as an extension number is subject to closed management in each PBX, and communication between the PBXs has such a form that control is performed through communication of limited information through a trunk line which interconnects the PBXs. Therefore, for connection between extension terminals accommodated in PBXs physically different from each other, call control processing must be performed separately on the transmission side and the reception side and also the exchanged information amount between the PBXs is limited. Consequently, employment of PBXs has a problem in that, when compared with a single PBX, service functions are limited significantly.
This similarly applies to operation of radio terminals accommodated in a private branch mobile communication system which is formed from radio base stations connected to such conventional PBXs. In particular, since management and operation information necessary for exchanging processing undergoes closed management in each PBX, the same SYS-ID cannot be provided to different PBXs. Consequently, use of a radio terminal is limited to the area of radio waves of radio base stations connected to a particular PBX in which attribute information of the radio terminal is registered for management and operation. And, in order to operate the radio terminal in the area of radio waves of a radio base station connected to a visitor PBX different from the home PBX in which the radio terminal is registered, roaming processing wherein the visitor PBX acquires operation data from the home PBX in which the radio terminal is registered and temporarily registers the radio terminal is required. In this manner, service functions are limited significantly. Also the hardware of the radio terminal must be constructed for roaming.
In particular, even if a private branch mobile communication system is constructed over a wide area using conventional PBXs, operation of each individual radio terminal is restricted significantly by a home PBX in which the radio terminal is registered. Consequently, and there is a problem that, while each radio terminal has portability as a radio terminal, it cannot sufficiently enjoy the benefit.
Further, even if a radio terminal moves between adjacent radio areas, if different SYS-IDs are used for the radio areas, then roaming processing must be performed, and there is a problem that a call in service is temporarily interrupted for such switching. Accordingly, even with a private branch mobile communication system wherein successive radio areas are formed over a wide area, where the radio areas are managed by individually different PBXs, roaming processing is performed upon movement a radio terminal between radio areas, and consequently, it is impossible to realize hand-over by which communication is continued between different radio areas.
It is an object of the present invention to provide a private branch mobile communication system and a private branch mobile communication method wherein, even where a local area switched network is formed from a plurality of PBXs and the individual PBXs manage attribute information of radio terminals independently of each other, a radio terminal used in a home PBX can perform, also in a radio area managed by a different PBX, continuation of communication or origination or termination of a call freely without performing roaming processing.
In order to attain the object described above, according to an aspect of the present invention, there is provided a private branch mobile communication system, comprising a distributed node switched network including a plurality of private branch exchanges each of which accommodates one or more radio base stations and has an identifiable node number provided thereto, and one or more radio terminals, a common system ID which defines a range of use over the distributed node switched network being allocated to the radio terminals, private branch exchanges and radio base stations, a network common database provided in each of the private branch exchanges and common for the radio terminals which have the system ID, and a private branch exchange individual database provided in each of the private branch exchanges for recording basic data of any of the radio terminals into the private branch exchange so that the private branch exchange may serve as a home node to the radio terminal, and means provided in each of the private branch exchanges for referring to the network common database to convert an ID from a radio terminal into a node number and a physical accommodation position provided by the private branch exchange of the home node to the radio terminal.
The private branch mobile communication system may further comprise a subscriber control section provided in each of the private branch exchanges for controlling communication with any of the radio base stations which belongs to the private branch exchange and performing fetching and forwarding of various events, a call control section provided in each of the private branch exchanges for discriminating which one of the private branch exchanges serves as a home node to that one of the radio terminals which has forwarded a position registration event by accessing the network common database of the private branch exchange itself and converting a unique ID of the radio terminal into a node number and a physical accommodation position, and a control signal transfer section provided in each of the private branch exchanges for transferring a control signal for rewriting the private branch exchange individual database of the private branch exchange of the home node in response to an instruction of the call control section.
The private branch mobile communication system may form a personal handyphone system.
According to another aspect of the present invention, there is provided a private branch mobile communication method, comprising the steps of forming a distributed node switched network from a plurality of private branch exchanges each of which accommodates one or more radio base stations and has an identifiable node number provided thereto, and one or more radio terminals, allocating a common system ID (SYS-ID) which defines a range of use over the distributed node switched network to the radio terminals, private branch exchanges and radio base stations, storing, into each of the private branch exchanges, a network common database common for the radio terminals which have the system ID and a private branch exchange individual database for recording basic data of any of the radio terminals into the private branch exchange so that the private branch exchange may serve as a home node to the radio terminal, determining, when any of the radio terminals which has the common system ID forwards a position registration request to one of the radio base stations which belongs to one of the private branch exchanges other than the home node to the radio terminal itself, by the private branch exchange to which the position registration request has been forwarded, an ID unique in the network of the radio terminal from the network common database and communicating, based on the ID unique in the network determined from the network common database and the node number, the position registration request of the radio terminal to the private branch exchange of the home node designated by the node number, and updating, by the private branch exchange of the home node, the position registration information of the private branch exchange individual database regarding the radio terminal.
With the private branch mobile communication system and method, even if a radio terminal which is communicating moves from a private branch exchange of a home node to the radio terminal itself to another private branch exchange, hand-over is possible without interrupting the communication, and origination or termination of a call can be performed freely through a plurality of private branch exchanges including the private branch exchange of the home node.
When the radio terminal performs a busy transfer request, data for busy transfer may be read out from the private branch exchange individual database of the private exchange of the home node to the radio terminal, and a communication path may be established between the radio terminal and an extension terminal of a destination of the transfer through the private branch exchanges. With the private branch mobile communication method, even if a radio terminal moves from a private branch exchange of a home node to the radio terminal itself to another private branch exchange, busy transfer is possible.
According to a further aspect of the present invention, there is provided a private branch mobile communication method, comprising the steps of forming a distributed node switched network from a plurality of private branch exchanges each of which accommodates one or more radio base stations, and one or more radio terminals, allocating a common system ID which defines a range of use over the distributed node switched network to the radio terminals, private branch exchanges and radio base stations, storing, into each of the private branch exchanges, a network common database common for the radio terminals which have the system ID and a private branch exchange individual database for recording basic data of any of the radio terminals into the private branch exchange so that the private branch exchange may serve as a home node to the radio terminal, and referring, when any of the radio terminals which has the common system ID performs a position registration event into one of the radio base stations which belongs to a second one of the private branch exchanges other than the home node to the radio terminal, by the second private branch exchange, to the network common database to access the private branch exchange individual database of the private branch exchange of the home node to perform position registration and reading out service data regarding the radio terminal stored in the private branch exchange individual data base of the home node, and then performing an additional service by the second private branch exchange.
With the private radio mobile communication method, even if a radio terminal is waiting at any physically different private branch exchange, communication is allowed with the private branch exchange of the home node, and there is an advantage that the radio terminal can always enjoy a service as an extension terminal of the private branch exchange of the home node.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference symbols.